Local Scala

To use a locally built Scala version, define the scalaHome setting,
which is of type Option[File]. This Scala version will only be used
for the build and not for sbt, which will still use the version it was
compiled against.

Example:

scalaHome := Some(file("/path/to/scala"))

Using a local Scala version will override the scalaVersion setting and
will not work with cross building.

sbt reuses the class loader for the local Scala version. If you
recompile your local Scala version and you are using sbt interactively,
run